1. Field of the Invention
This invention relates to polarizing films containing a photodichroic material.
In recent years, polarizing films are being widely used as an element of liquid-crystal display devices for, for example, electronic computers and watches or as glare shields applied to, for example, glass. However, the development of the electronics industry has enlarged the field of application of liquid-crystal display devices, leading to a demand for improved performance of liquid crystals per se. As a result, it has also come to be required that polarizing films used in combination with liquid crystals should attain an improved degree of polarization, exhibit optical uniformity, and have excellent thermal resistance and moisture resistance.
2. Description of the Prior Art
Conventionally, polarizing films comprising polyvinyl alcohol (PVA) and iodine are commonly used because of their good polarizing properties. However, polarizing films of the PVA-iodine type have the disadvantage of being insufficient in moisture resistance and thermal resistance. Accordingly, durable polarizing films comprising a thermoplastic resin and a photodichroic material are now being developed.
Nevertheless, polarizing films comprising a thermoplastic resin and a photodichroic material fail to exhibit satisfactorily high polarizing power and, therefore, have not yet been put to practical use.
There are two well-known methods of making polarizing films containing a photodichroic material. One method involves impregnating a uniaxially oriented film of base resin with a photodichroic material such as iodine or a photodichroic dye, while the other involves uniaxially orienting a film of base resin containing a photodichroic material. In either of these methods, the base resin is uniaxially oriented to arrange its molecules in one direction and the photodichroic material is oriented on the basis of the orientation of the base resin to manifest its polarizing power. Accordingly, it is believed that there is a close relationship between the state of orientation of the base resin and the polarizing power of the film. Usually, the state of orientation of a resin is evaluated by determining its uniaxial orientation coefficient (f), and greater values for f indicate higher degree of orientation. Thus, it is presumed that the polarizing power of a film is enhanced as its degree of orientation becomes higher.
However, in the course of research in polarizing films using a hydrophobic base resin and containing a photodichroic material, the present inventors have found a phenomenon which cannot necessarily be accounted for by the above-described principle. More specifically, it has been found that, in order to obtain a film having good polarizing power, the film must have not only a high uniaxial orientation coefficient (f) as determined by the X-ray diffraction method, but also good uniformity of diffraction intensity on the Debye-Scherrer ring produced by the X-rays imping parallel to the reference axis on the specimen which is cut out perpendicular to the reference axis of the film. (The reference axis will be described later.)